Glass bottles are formed in a process which can produce defects which make the bottle unacceptable. For example, the side wall of a bottle may include a “stone” which makes the bottle visually unacceptable or the bottle may have a “bird swing” which could break off and become part of the contents of the bottle.
One of the crudest ways that a bottle is inspected is for an operator to remove a bottle from a conveyer and look through the bottle at a light source within sight of the operator (a ceiling light for example). Operators believe that certain defects can be identified in this manner. Inspection equipment improves on this process and conventionally defines a light source which is part of an inspection station within a piece of inspection equipment. The inspection may be of a very simple form with an inspector watching bottles as they pass in front of a large opaque light screen but this light source may be inconvenient for the operator.
A camera may look through a bottle which is backlighted by this light source and an inspection algorithm attempts to identify a defect. The back light may have a great variety of forms but defines either a diffuse back light or a patterned back light. One form of patterned back light is a black and white stripe such as would be defined by a light intensity having an intensity which varies linearly as a sine function.